Sleep and wakefulness were recorded in cats after oral administration of the 5-HT2 antagonist ritanserin at doses of 0.5 and 1.0 mg/kg (n = 6), and after 2.5 mg/kg (n = 3). A subgroup (n = 4) of the animals also received the selective 5-HT uptake inhibitor zimeldine and a combination of ritanserin and zimeldine. The drugs were administered shortly after light onset, and sleep was recorded for 15 hours. Waking was increased and slow wave sleep (SWS), particularly deep slow wave sleep (SWS-2), was decreased throughout the recording time after ritanserin administration. Rapid eye movement (REM) sleep latency was increased, and the amount of REM sleep was reduced. In the combination study, ritanserin reduced the delayed SWS increase following the serotonin uptake inhibitor. The findings of increased waking and decreased SWS-2 after ritanserin in cats contrasts with findings from humans and rats where slow wave sleep is increased following ritanserin administration.